If all you want to do is change the .cshtml of those pages, you can simply override them. To do this, copy the templates as follows:
- LogOn.cshtml and Register.cshtml from Orchard.Users/Views to [YourTheme]/Views
- Everything from Orchard.Users/Views/Account to [YourTheme]/Views/Orchard.Users/Account
(LogOn.cshtml and Register.cshtml are shapes, but all the other files are normal MVC views, which is why the override locations are different)
Now you have a copy of all the relevant files so you can make any changes directly in your theme.
If you want to override the actual behaviour of a specific page it gets more tricky, but again tell us exactly what you want to do, there's already a module available for registration customisation which might help you.